Surface plasmon sensor for analyzing liquid sample or humid atmosphere

A surface plasmon sensor which can be placed under a liquid sample or in a humid atmosphere during operation. The surface plasmon sensor is constituted of a dielectric block 11, a metal film 12 which is superposed on one surface of the dielectric block 11, a light source 14 for generating a light beam 13, a front optical system 15 which enables the light beam 13 to realize a variety of angles of incidence on an interface 11a between the dielectric block 11 and the metal film 12, each of the angles satisfying a condition for total reflection on the interface 11a, including those angles satisfying a condition for the surface plasmon resonance, and a photodetector 16 which detects a state of the surface plasmon resonance by measuring intensity of the light beam 13 after the light beam 13 is totally reflected at the interface 11a. The dielectric block 11, the light source 14, the front optical system 15 and the photodetector 16 of the surface plasmon sensor are housed in a box 20 having a waterproof structure keeping the metal film 12 on the dielectric block 11 in contact with an external atmosphere.